I really find this question interesting and I would like to know the reason myself. And I would like to know in fact: are they really proper nouns (PN)? The definition of PN is not the same in all the languages. Even within a given language the definition of PN is not usually very clear. In Spanish or Italian the days of the week, the names of the months, and the names of the seasons are not capitalised. However, the Real Academia Española doesn't say they are not PN. Nevertheless a spelling rule in Spanish says that "all proper nouns must be capilalised" so we just assume they are not PN.
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user5660Mar 3 '11 at 9:25

4 Answers
4

A proper noun names a specific member of a group: Janet, Asia, and Cadillac are proper nouns. Proper nouns are always capitalized.

This is what I find in English Grammar (ISBN 0-06-467109-7).

Asia is a member of the group of the continents, in the same way January is a member of the group of months, and Monday is a member of the group of weekdays.
As per definition of proper nouns, weekday names are proper nouns.

red, green, and blue are members of the group of colors, but they're not proper nouns. I find this rule insufficient.
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moiociAug 21 '10 at 4:05

Red, green, and blue are also adjectives, while Jane is only a noun.
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kiamlalunoAug 21 '10 at 4:12

11

Autumn, spring, summer and winter are the names of seasons, but they're not used as proper nouns, except when personified in literature.
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TragicomicFeb 1 '11 at 11:45

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I was brought up in the US and I always thought that the names of the seasons were capital when you were referring specifically to that thing, but not when they're used as adjectives. "Soon it will be Autumn" vs. "Soon it will be time for the autumn harvest". I know that they're never capitalised in British English though.
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mkoistinenNov 19 '13 at 18:22

True for most, but not Sunday and Monday -- perhaps these two ended up capitalized because the majority of days were?
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KosmonautAug 18 '10 at 15:39

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@Kosmonaut: while I am not defending Mehper's hypothesis (frankly, I have no idea what the correct answer is), I do have to point out that both the Sun and the Moon are often capitalized, too, when referring to, well, the Sun and the Moon as opposed to a star and some planet's satellite. I would thus argue that the names of all seven days were created equal, rather than ended up being equal as you suppose.
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RegDwigнt♦Aug 19 '10 at 13:13

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@RegDwight Good point. Going on that fact, another interesting thing (in terms of capitalizing words) is that the words for Sunday and Monday in French, Spanish, and Italian are all lowercase, even though the words for the Sun and Moon themselves are capitalized.
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KosmonautAug 19 '10 at 13:59

@Kosmonaut: That is because in Italian we don't always write a proper name capitalized; In example, we write "the Italians", but "italian men".
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kiamlalunoAug 19 '10 at 14:34

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@ kiamlaluno Actually, in that case it is the difference between a proper noun and an adjective ("the Italians" is a noun, "italian" is an adjective). German does the same -- "the Germans", but "the german Men", even though they capitalize ALL nouns. (In fact, English is the only writing system I know of that attempts to capitalize some adjectives coming from proper nouns.) But the days of the week are clearly nouns.
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KosmonautAug 19 '10 at 14:47

The days in Latin were related to the Mesopotamian days, taken from seven celestial objects (the Sun, the Moon and planets). Those celestial objects have a name, so the names are nouns.

I read about the etymology statement, and I really don't know if it has nothing to do with this, but as the question says, as I recall the proper nouns in Spanish are called "Nombres Propios", something like personal names that clears a lot the idea behind them.

Proper nouns turned into massive usage, don't capitalize "aspirin". There's a lot of brands of aspirin, so even when it was originally a proper noun it became common.

Demonyms or Gentilic, in Spanish, are always written non-capitalized. In your case, capitalized. (Demonym and change from a Spanish to an English idiom.)

Finally, in the case of the days of the week, originally capitalized (proper nouns), discussed by the Spanish Language Royal Academy (RAE) became non-capitalized by the same reasons aspirin is not capitalized. But this happened about 10 years ago(?) I don't know, but I think your question has no proper answer in the light of the evidence of other cases in another idioms. Like the one exposed in Spanish.

What are the 4-5 columns in the table? The third seems to be English. Is the first Spanish? And what are the others?
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Peter MortensenAug 26 '11 at 7:00

Spanish day of the week - Spanish celestial object - English celestial object - what I assume is Latin day of the week. Saturday and Sunday are odd-balls because the Spanish day names don't follow the Latin pattern, and instead come from words meaning Sabbath and Lord's day. He seems to have vacillated a bit about how to represent this.
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WlerinAug 31 '14 at 1:20